The invention relates to an arrangement and method for connection or call admission control (CAC) in a broadband network. The invention further relates to software for carrying out the method.
Broadband, e.g. ATM networks are being increasingly used for the transport of narrow band voice traffic, typically by interfacing the broadband network with one or more legacy TDM voice networks. In order to adapt this narrow band traffic to the ATM transport protocols, it has become common practice to package this traffic into minicells using the ATM Adaptation Layer 2 (AAL2) protocol. These minicells are then transported within ATM cells.
It will be appreciated that one of the constraints placed on the transport of voice traffic is the provision of guarantees on the quality of service (QoS). This in turn requires appropriate network management to ensure that sufficient bandwidth to support the quality of service is, and will remain, available before a call or connection request is accepted. This process is generally referred to as connection admission control.
Connection admission control is one of a number of known techniques for managing and controlling traffic and congestion in, connection-orientated networks. In particular, it is used in ATM (asynchronous transfer mode) networks to provide quality of service (QOS) guarantees. It is not of course limited to use in ATM networks. Connection admission control (CAC) procedures are used to decide if a request for an ATM connection can be accepted, based on the network capacity and the attributes of both the requested connection and existing connections. This is one application which requires that an equivalent bandwidth be determined accurately both for the new connection and for the existing connections. It is important that there is always enough bandwidth so that quality of service guarantees for the existing connections and the requested connections, can be met.
Connection admission control (CAC) procedures may be used at an access node at the edge of an ATM network to enable control of access to the route through the ATM network as route selection is made. A second level, may be used at each node along the selected route through the ATM network, to confirm that a respective link beyond that node, can admit the connection.
An estimate of the bandwidth required by the connection, and knowledge of the available bandwidth on each link is required. The CAC algorithm at the network edge uses parameters available from the routing database, and characteristics of the connection being requested (available from signalling information) to determine if an individual link is likely to accept or reject the connection. The link/node is included if it is likely to accept the connection, and excluded from the route selection algorithm if it is unlikely to accept the connection.
After path selection is done, each node along the chosen route executes its own CAC algorithm, using factors such as link capacity, buffering capability or queuing architecture, traffic descriptors, QOS requirements and capacity allocated to different types of traffic or different connections.
Some of these parameters are fixed and some are variable. Queue size and the desired QOS are examples of fixed parameters, whereas the traffic descriptor and current available link capacity are dynamic parameters. The calculation is complex because connections typically use variable rates of ATM cell flow. Such flows can be described statistically using parameters such peak cell rate, and maximum burst size. By calculating an effective capacity, also known as effective bandwidth, for individual connections, many connections can share the bandwidth of an individual link more efficiently, without having to provide the peak bandwidth for all connections.
Many algorithms have been proposed for determining the effective capacity of the requested connections, and of existing connections. Some are described in an article entitled xe2x80x9cPerformance Evaluation of Connection Admission Control Techniques in ATM Networksxe2x80x9d by Jamoussi et al, published in a 1996 IEEE journal. This article notes that a good CAC algorithm strives to achieve a balance of the objectives of QOS guarantee, execution speed, link efficiency, and simplicity.
A summary of admission techniques is provided by Perros and Khaled in IEEE communications magazine November 1996, xe2x80x9cCall Admission control schemes, a reviewxe2x80x9d. One known technique is shown in an article by Guerin et al entitled xe2x80x9cEquivalent capacity and its application to bandwidth allocationxe2x80x9d from the IEEE journal on selected areas in communications. Vol 9, no. 7. It involves determining an approximation for the equivalent bandwidth of an individual connection by using a known relationship between parameters of the connection, size of buffer at the Admission control node, and a quality of service matrix which may be probability of overflow, i.e. cell loss ratio (CLR).
The above referenced paper by Guerin et al shows that an approximation based on a combination of a fluid flow approximation and a stationary or static approximation gives results to an exact evaluation of equivalent capacity as defined by equation (1).                               ∈                      =                                          β                ·                exp                            ⁢                              xe2x80x83                            ⁢                              (                                  -                                                            K                      ⁢                                              xe2x80x83                                            ⁢                                              (                                                  c                          -                                                      ρ                            ⁢                                                          xe2x80x83                                                        ⁢                                                          R                              peak                                                                                                      )                                                                                    b                      ⁢                                              xe2x80x83                                            ⁢                                              (                                                  1                          -                          ρ                                                )                                            ⁢                                              xe2x80x83                                            ⁢                                              (                                                                              R                            peak                                                    -                          c                                                )                                            ⁢                                              xe2x80x83                                            ⁢                      c                                                                      )                            ⁢                              xe2x80x83                            ⁢              where                                      ⁢                  
                ⁢                  β          =                                                                      (                                      c                    -                                          ρ                      ⁢                                              xe2x80x83                                            ⁢                                              R                        peak                                                                              )                                +                            ∈                              ρ                ⁢                                  xe2x80x83                                ⁢                                  (                                                            R                      peak                                        -                    c                                    )                                                                                    (                                  1                  -                  ρ                                )                            ⁢                              xe2x80x83                            ⁢              c                                                          (        1        )            
Rpeak=Peak rate
xe2x96xa1=Probability of overflow (i.e. CLR)
c=Equivalent capacity
K=Buffer size
xe2x96xa1=Fraction of time source active
b=Mean duration of active period (talk spurt)
As this equation is computationally extremely strenuous the approximation using the minimum of the fluid flow approximation and the stationary approximation as proposed by Guerin and shown in equation (2) is used.                     C        =                  min          ⁢                      {                                                                                ∑                                          t                      =                      1                                        N                                    ⁢                                      xe2x80x83                                    ⁢                                      m                    i                                                  +                                                      a                    xe2x80x2                                    ⁢                                      xe2x80x83                                    ⁢                                                                                    ∑                                                  i                          =                          1                                                N                                            ⁢                                              xe2x80x83                                            ⁢                                              σ                        i                        2                                                                                                        ,                                                ∑                                      i                    =                    1                                    N                                ⁢                                  xe2x80x83                                ⁢                                  c                  j                                                      }                                              (        2        )            
where
xcex11={square root over (xe2x88x922l n(xcex5)xe2x88x92ln(2xcfx80))}
mxe2x96xa1=Mean bit-rate of ith source
Rpeaki=Peak rate of ith soure
xe2x96xa1C2=Variance of ith source, =mix(Rpeakixe2x88x92mi)
xe2x96xa1=Probability of overflow (i.e. CLR)
Ca=the total equivalent bandwidth of N channels
As can be seen, this equation (2) is based on values of mean bit rate. Variance, and bandwidth used (otherwise termed equivalent capacity). These values are determined by adding look up values which represent the increment or delta beyond the current running totals of these values, to the existing running totals. The resulting values are used to calculate static and flow approximations and thus the resulting equivalent bandwidth to be in use if the requested connection is to be admitted.
The relationship defined in equation (1) above is complex, and so can only be evaluated by numerical or iterative methods which are too computationally intensive to be usable in a practical network with sufficient accuracy. Accordingly, in Guerin et al, a major factor in this complex relationship, is approximated rather than evaluated. This enables the relationship to be evaluated using normal algebraic methods without requiring a lengthy numerical analysis or iterative method.
To calculate the aggregate equivalent bandwidth of the numerous connections already admitted, so that the available bandwidth can be determined, Guerin et al proposes taking the minimum of two approximations. The first is a static approximation, and the second is a fluid flow approximation. The result is greater than the real equivalent capacity, except where static evaluation under-estimates the equivalent capacity. The static approximation is representative of the bandwidth required for a large number of connections, when the effects of statistical multiplexing become significant. The fluid flow approximation is more representative of actual connection behaviour and so is more useful when the number of connections is small. The static approximation is the sum of the mean bandwidths of the individual connections, which can be measured or estimated, plus a proportion of the standard deviation of the aggregated connection. The proportion reflects the desired quality of service, or risk of dropping part or all of the information flow of one or more connections.
The fluid flow approximation is more difficult to calculate. In Guerin a straightforward summation of the values calculated for individual connections is made. This implies an assumption of a linear relationship between the equivalent bandwidth and the number of connections. In Guerin et al different equivalent capacity values can be calculated for each of several different traffic classes, hence the equivalent bandwidth will depend on both the number of connections, and the traffic class. Nevertheless, the relationships remain linear.
Reference is also here directed to our co-pending applications, Ser. Nos. 09/368,276 and 09/368,278.
A further problem in determining the network parameters in order to provide an efficient connection admission control has arisen in networks that serve a number of small business users who may have a small number of lines served e.g. via a PBX and which share a common virtual circuit connection. These sources can have low traffic flows in which cells transporting the traffic are only partly filled and which exhibit a significant departure from the standard traffic flow model. Such traffic flows are not readily amenable to the calculation of bandwidth requirements using present connection admission control algorithms. To ensure the delivery of a defined quality of service, it is currently necessary to over-estimate the bandwidth demands of such traffic. This leads to a less than efficient use of network resources.
An object of the invention is to minimise or to overcome the above disadvantage.
A further object of the invention is to provide an improved arrangement and method for connection admission control in a communications network.
According to a first aspect of the invention, there is provided a method for controlling admission of connections to a network from a plurality of sources sharing a common virtual channel connection in which information packets are carried in ATM cells, the method comprising:
receiving a request for admitting a connection to the network,
determining whether that connection requires the transmission of a single packet per cell, partially filled cells containing more than one packet or fully filled cells,
where a single packet is to be transmitted per cell, calculating a bandwidth requirement via a first algorithm,
where fully filled cells are to be transmitted, calculating the bandwidth requirement via a second algorithm,
where partially filled cells containing more than one packet are to be transmitted calculating the bandwidth requirement via an interpolation between said first and second algorithms, and
determining whether to admit the requested connection according to whether a bandwidth used would exceed a maximum bandwidth allowed for at least a portion of the network.
According to another aspect of the invention, there is provided an arrangement for controlling admission of connections to a network from a plurality of sources sharing a common virtual channel connection in which information packets are carried in cells, the arrangement comprising:
means for receiving a request for admitting a connection to the network,
first decision means for determining whether that connection requires the transmission of a single packet per cell, partially filled cells containing more than one packet or fully filled cells,
first calculation means for calculating a bandwidth requirement via a first algorithm where a single packet is to be transmitted per cell,
second calculation means for calculating a bandwidth requirement via a second algorithm where fully filled cells are to be transmitted,
third calculation means for calculating a bandwidth requirement via an interpolation between said first and second algorithms where partially filled cells containing more than one packet are to be transmitted, and
second decision means for determining whether to admit the requested connection according to whether a bandwidth used for that connection would exceed a maximum bandwidth allowed for at least a portion of the network.
According to a further aspect of the invention, there is provided a communications network arrangement comprising; an asynchronous transfer mode (ATM) core network, a plurality of gateways providing access to said core network, a plurality of sources sharing a common virtual channel connection in which information packets are carried in ATM cells, and connection admission control means for controlling admission of connections to the core network from said sources, wherein the connection admission control means comprises;
means for receiving a request for admitting a connection to the network,
first decision means for determining whether that connection requires the transmission of a single packet per cell, partially filled cells containing more than one packet or fully filled cells,
first calculation means for calculating a bandwidth requirement via a first algorithm where a single packet is to be transmitted per cell,
second calculation means for calculating a bandwidth requirement via a second algorithm where fully filled cells are to be transmitted,
third calculation means for calculating a bandwidth requirement via an interpolation between said first and second algorithms where partially filled cells containing more than one packet are to be transmitted, and
second decision means for determining whether to admit the requested connection according to whether a bandwidth used for that connection would exceed a maximum bandwidth allowed for at least a portion of the network.
According to another aspect of the invention, there is provided software on a computer readable medium for controlling admission of connections to a network from a plurality of sources sharing a common virtual channel connection in which information packets are carried in cells, wherein the method comprises the steps of:
receiving a request for admitting a connection to the network,
determining whether that connection requires the transmission of a single packet per cell, partially filled cells containing more than one packet or fully filled cells,
where a single packet is to be transmitted per cell, calculating a bandwidth requirement via a first algorithm,
where fully filled cells are to be transmitted, calculating the bandwidth requirement via a second algorithm,
where partially filled cells containing more than one packet are to be transmitted calculating the bandwidth requirement via an interpolation between said algorithms, and
determining whether to admit the requested connection according to whether a bandwidth used would exceed a maximum bandwidth allowed for at least a portion of the network.